As is well known, in several integrated circuit configurations, it is desirable to utilize voltages which are negative with respect to the ground GND of the circuit. Such voltages are produced on the chip by means of so-called charge pumps. A typical example of such applications is that of the DRAM memories and of the Flash EEPROM memories.
If it were possible to switch such negative voltages, the performance of these circuits, and in particular of Flash EEPROM memories, would be noticeably improved. In N-well CMOS technology, the N-channel devices cannot be used to this end because they are incorporated in the P-type substrate of the integrated circuit and therefore their source/drain terminals cannot be exposed to negative voltages, so as not to forward bias P-N junctions, which would cause the break-down of the negative voltage generator (charge pump).
There are many circumstances in which the integrated circuit applications in CMOS technology would be noticeably improved, if it were possible to selectively switch a negative voltage.
For instance, a Flash EEPROM memory, which is normally erased as a whole by means of a negative voltage produced at the interior of the chip, could be operated as a standard EEPROM memory, if it were possible to selectively apply the negative voltage to a single row or to a single word of the memory. Another example could be given by the implementation of a multiple state logic, in particular a tristate logic +1, 0, -1 in which the logic state -1 would be associated with a negative voltage.
The present CMOS technologies are in most cases based upon N-channel devices which are incorporated on a common P-type substrate (coinciding with the base material of the integrated circuit and connected to the ground GND), while P-channel devices are fabricated inside purposely diffused N-type regions (N-wells). Consequently, P-type devices may rely upon independent substrates, while this possibility does not exist for N-channel devices.
A locally generated, positive voltage V.sub.pp, higher than voltage V.sub.cc, may be easily switched selectively by using a decoding network made up of N-channel transistors and of P-channel transistors, where the substrates of the P-channel devices are biased to V.sub.pp. In theory, it should also be possible to selectively switch a locally produced negative voltage -V.sub.nn lower than the ground voltage GND, but in practice this is not feasible, because of the impracticality of connecting the substrates of the N-channel transistors to the voltage -V.sub.nn.
Clearly, the problem is not that of producing a voltage lower than the ground voltage GND on the chip by means of charge pumping techniques using only P-channel transistors and capacitors. However, in CMOS technology, present day solutions do not selectively route the negative voltage to the various portions at the interior of the chip.
Therefore, it may be seen that a need has arisen in the industry for a method and apparatus for selectively switching a negative voltage (-V.sub.nn) to portions of CMOS integrated circuits.